1. Field of the Invention
The present invention relates to the microbiological nitrification and denitrification of wastewater.
2. The Prior Art
Nitrogen, a component of domestic wastewater, exists in several forms, all of which are cause for concern when discharged to the environment in an uncontrolled manner:
1) The nitrogen in ammonia (NH.sub.3) exerts an oxygen demand on the water in which it is discharged, and is known to be toxic to many forms of aquatic life;
2) The nitrogen in nitrite (NO.sub.2) is toxic if ingested by vertebrate forms of life. Because it is unstable in solution it is usually not of public health significance;
3) The nitrogen in nitrate (NO.sub.3) is believed to be toxic to infants under the age of 6 months, causing a condition known as infant methemoglobinemia, wherein chemical conditions unique to the baby's stomach convert the nitrate to nitrite, which then exerts its toxic effect and interferes with the role of hemoglobin in human respiratory and metabolic function;
4) All forms can act as a plant nutrient, stimulating undesirable growth of algae and other aquatic plant life.
Conventional secondary wastewater treatment generally leaves soluble forms of nitrogen unaffected. Some reduction in total nitrogen is achieved, because forms known as organic nitrogen that are particulate or colloidal are generally coagulated and removed by sedimentation. In more recent years, particularly the last 20 years, some wastewater treatment plants have been built to achieve nitrification (the conversion of ammonia to nitrate), in order to remove the oxygen demand that ammonia exerts on receiving waters. More recently, wastewater treatment plants have been designed to achieve total nitrogen removal in order either to protect drinking water supplies from the potential of nitrate levels exceeding drinking water standards, or to protect sensitive watersheds from nitrogen related eutrophication.
In order for nitrogen to be removed biologically, the following steps are required:
1. In a first step, organic matter in the sewage is oxidized in the presence of dissolved oxygen by heterotrophic bacteria (secondary treatment);
2. In a second step, in the same reactor, or possibly a successive reactor, autotrophic bacteria oxidize ammonia nitrogen into nitrite and then into nitrate nitrogen;
3. In a third step, additional heterotrophic bacteria use the nitrate as an oxygen source in order to oxidize and consume additional organic material. This step takes place in the absence of dissolved oxygen, which forces organisms to turn to nitrate as the source of oxygen.
A variation on the third step uses sulfur instead of organic carbon. Instead of heterotrophic (carbon consuming) bacteria, a separate culture of autotrophic sulfur oxidizing bacteria, i.e. Thiobacillus denitrificans uses the nitrate to oxidize sulfur.
Steps 1, 2 and 3 described above have the disadvantage of requiring the procurement of a separate source of carbon for the denitrification step. This is necessary because the oxidation taking place in Steps 1 and 2 consumes the bulk of the available organic carbon present in the wastewater, and there is not sufficient carbon left for the nitrate reduction in Step 3. There is an additional disadvantage in the sequence described above in that supplemental alkalinity in the form of bicarbonate, carbonate or hydroxide may have to be added in order to maintain an acceptable reaction rate during Step 2.
A variation in this prior art process can be achieved by changing the sequence of the steps listed above, into a process sequence called "Pre-Denitrification." The most basic means of accomplishing this is to put the denitrification step at the beginning of the process. Since no nitrate exists in the raw wastewater, nitrate is supplied to the denitrification step or zone by recirculating nitrified effluent. There are several prominent characteristics to this flow scheme:
1. The significant expense of purchasing sulfur, methanol, or a carbon source other than methanol is eliminated;
2. Denitrification, based on the chemistry of the reactions, releases approximately 50% of the alkalinity that was previously consumed during the nitrification process. In many or most cases, this is a sufficient quantity so that supplemental alkalinity addition is not required;
3. The oxygen present in the nitrate being denitrified is being recycled for the purpose of re-oxidizing organic carbon present in the raw wastewater.
Prior art systems tend to fall into two categories:
1) Devices for the purpose of achieving nitrification or denitrification in modular steps in the conventional flow sequence; and
2) Systems for achieving pre-denitrification by means of recirculating nitrified effluent to a denitrification step at the upstream end of the process.
The use of recirculation occurs in all of these devices, but it must be noted that the purpose of the recirculation may be completely different from one device to the next. This will be discussed in this section since it is felt that it is important when determining differences or similarities between systems.
Neff, U.S. Pat. No. 3,994,803, relates to a filter for accomplishing denitrification with methanol as a source of carbon. This is an example of denitrification as the final stage of a process. The particulate medium in the filter (sand and anthracite) provides a population of heterotrophic bacteria which utilize the nitrate as an oxygen source when consuming the methanol. This filter is always submerged, and periodically must be backwashed in order to remove excess bacterial growth from the filter medium. Failure to do so would result in excessive head loss. Furthermore, nitrogen gas, a by-product of denitrification, builds up in the filter and this must also be purged to prevent excessive headloss from occurring. At the beginning of the filtration cycle following backwash, some dirty backwash water is recirculated to the filter. The purpose of this recirculation is to reseed the filter with the bacteria necessary to accomplish the desired denitrification. This is felt to be necessary because vigorous backwash may strip too much of the viable bacteria off of the sand or anthracite grains in the filter.
Recirculation sometimes plays a role in certain activated sludge processes. Its purpose is to return settled microorganisms ("activated sludge") from the settling tank back to the aerated biological process.
When pre-denitrification is practiced, an additional "internal recirculation stream" is added, returning nitrified activated sludge from the aerated nitrifying zone or reactor to the anoxic or anaerobic denitrifying zone. If all the nitrate entering the denitrifying zone is denitrified, and if all the ammonia entering the nitrifying zone is nitrified, the expected final nitrate (and soluble nitrogen) concentration is (Ammonia in) ##EQU1## where Q is the raw wastewater flow rate and .SIGMA.R is the sum of recirculation flow rates. This expression shows that the degree of nitrogen removal in a pre-denitrification system is dependent on recirculation flow rate.
Fixed film biological filters, such as trickling filters, fluidized bed reactors and other devices, also utilize recirculation.
Some fixed film reactors, where water is the dispersed phase, such as the trickling filter or recirculating sand filter, use recirculation to (1) ensure that the media is thoroughly wetted; and (2) add dissolved oxygen to the applied liquid.
In order to achieve consistent nitrification, these filters require low organic loading rates. If the wastewater is fairly strong, the corresponding hydraulic loading rate will be low, and may not be sufficient to ensure contact of the applied liquid with all of the media. This problem is solved by recirculating clear effluent, which is low in organic matter, permitting better wetting of the media while keeping organic loading low.
In some cases, dissolved oxygen availability may limit the degree of biological activity. Recirculation aids here, also. In a continuous flow system, such as a trickling filter, research over many years has shown that the rate of absorption of a gas into a liquid flowing over packing is a function of liquid flow rate, frequently proportional to the 0.5-0.8 power. Hence, quadrupling flow rate would double oxygen availability (for exponent=0.5). In an intermittent flow system, such as a sand filter, pump flow rate would be constant, but recirculation would increase the pump running time, thereby increasing proportionately the opportunity for oxygen uptake.
The role of recirculation is similar with fluidized bed systems, with some differences due to the fact that the liquid phase is now the continuous phase:
1. Recirculation controls the degree of be expansion, and the degree of fluid shear against the organisms on the media, which controls biofilm thickness and particle density; and
2. Recirculation water is the means for adding dissolved oxygen to aerobic processes, such as nitrification and organic carbon oxidation. In this application, the recirculation stream is aerated with a compressor prior to reinjection into the column.
In the past, other attempts have been made to overcome these prior art problems, as follows.
Bordigoni, U.S. Pat. No. 853,217, describes a septic tank followed by an upflow anaerobic filter, followed in turn by a downflow aerobic filter. Flow is on a once-through basis without recirculation. Dosing is strictly by gravity.
MacCormac, U.K. Patent No. 26013, describes a settling compartment followed by an anaerobic upflow filter, followed by an aerobic downflow filter. Flow is on a once-through basis and dosing is by siphon.
K. A. Porter, et. al., U.S. Pat. No. 3,112,261, describes an anaerobic biological filter using a lattice grid type medium. Dosage is pumped to the filter and recirculation is used in order to enhance oxygen transfer to the fluid.
Simmons, et. al., U.S. Pat. No. 3,371,033, describes a combination of aerobic activated sludge plus a biological filter. Recirculation is practiced for the purpose of returning so ids to the activated sludge portion of the process as well as maintaining elevated dissolved oxygen in the filter.
Matsch, et. al., U.S. Pat. No. 4,173,531, describes a process for nitrifying activated sludge with a side stream recirculated through a fairly long anaerobic holding process. Return of this stream to the upstream end of the activated sludge aeration basin is alleged to result in far greater nitrogen removal than the material balance for pre-denitrification would predict.
Fan, et. al., U.S. Pat. No. 4,322,296, describes a fluidized bed biological filter. The top plate restricts the degree to which the bed can rise and turns the upper portion of the reactor into a fixed bed filter. This prevents media from escaping the device and eliminates the need for a machine to clean escaped media and return it to the filter. As a result, backwash cycles are required. Recirculation is practiced for the purpose of controlling fluidization velocity and also for adding dissolved oxygen for aerobic processes. The device can be used for either aerobic or anaerobic processes, i.e., either carbon removal, nitrification or denitrification.
Zorich, et. al., U.S. Pat. No. 4,895,645, describes a packaged biological filter wherein pre-denitrification and anaerobic carbon removal take place in an upflow rock filter, while nitrification takes place in a downflow plastic filter. Nitrified plastic filter effluent is recirculated to the anaerobic rock filter along with the raw wastewater, making this a fixed media pre-denitrification system. Recirculation, therefore, is used both for wetting of the plastic trickling filter media and for furnishing nitrate to the anaerobic filter. Two separate recirculation streams are required, but can be furnished by one pump if flow is split.
Lagana, et. al., U.S. Pat. No. 4,915,841, describes an anaerobic suspended growth reactor located in the lower portion of an Imhoff tank, followed by an anaerobic upflow biological filter, followed by an aerobic downflow biological filter (the liquid can be either continuous or discontinuous as desired in the last zone). Recirculation is used both for pre-denitrification and (in the discontinuous phase filter) for the wetting of the aerobic filter. A single recirculation stream to the upstream end of the plant serves both functions.
Gotz, U.S. Pat. No. 5,049,266, describes a pre-denitrification system composed of an anaerobic upflow filter followed by an aerobic downflow filter. Recirculation is used to accomplish pre-denitrification. In addition, the downflow filter is of the trickling filter type, i.e. water is not the continuous phase, so recirculation also serves for filter wetting and oxygen transfer.
Other examples of wastewater treatment process include those described in Porter, U.S. Pat. No. 3,112,261; Hashimoato, U.S. Pat. No. 3,829,377; Japanese Patent No. 1099-690A; French Patent No. 1,110,962; German Application No. 3,431,568; and German Application No. 3,419,139.
Nitrate pollution of groundwater is a problem frequently associated with discharges from septic tank leach fields. Bacteria in the soil convert (oxidize) ammonia nitrogen in the wastewater to nitrite and then to nitrate. Nitrate (as nitrogen) in excess of 10 mg/1 (ppm) is considered to be a risk factor for methemoglobinemia in infants and certain other susceptible populations, as discussed supra.
In some areas, such as the New Jersey Pinelands, the nutrient value of nitrogen in septic tank effluent is a concern, based on its ability to fertilize nearby surface waters.
As a result of the above, planning documents such as the Long Island 208 Study have recommended control of zoning density based on nitrogen loading per acre.
The Long Island Groundwater Pollution Study (1969) clearly demonstrated that raw wastewater was fully nitrified and well clarified after passing through only several inches of Long Island sand and gravel beneath a leaching pool such as a cesspool.
Mulbarger et al have demonstrated in numerous instances that a carbon substrate, such as methanol, could be used to achieve denitrification (reduction of nitrate to nitrogen gas) in a low oxygen environment.
Lawrence, at Cornell University (JWPCF, 1970), demonstrated that Thiobacillus dentrificans with sulfur substrate and limestone supplement could reduce nitrate to nitrogen gas (used powdered sulfur in agitated column).
Suffolk County Department of Health Services (unpublished study) successfully repeated the work of Lawrence with granular materials in a packed column.
Andreoli, Bartilucci and Forgione (JWPCF, 1979) conducted an experiment wherein a septic tank leach field was underlain by a fiberglass pan which would catch effluent and maintain saturated conditions (in the pan) necessary for a low oxygen environment. Methanol was added to the pan. Nitrification occurred in the leach field and denitrification occurred in the pan.
Suffolk County Department of Health Services (1984) developed a subsurface denitrification design that consisted of gravity feed to septic tanks, gravity feed to leaching pools, leaching through two feet of sand (nitrification), collection in a vinyl (PVC) liner, and passage through an upflow (sulfur and limestone) reactor (denitrification).
This prior art design has undergone several modifications: (a) leaching pools eliminated, only horizontal perforated pipes allowed for dosing, vertical travel through sand increased to four feet; and (b) gravity dosing eliminated, all distribution is now pumped, but main is not pressurized. All hydraulic loadings at one gallon per day per square foot of leaching area.
Swanson, Dix (EPA Small Flows Clearinghouse, Morgantown, W. Va.) and others have demonstrated the effectiveness of recirculating filters for nitrification.
This prior art system has produced disappointing results in numerous installations. The majority of systems do not achieve the needed reduction from 40-50 mg/1 total N down to 10 mg/1 total N.
Most prior art failures appear to be due to lack of nitrification. Some prior art failures are caused by lack of denitrification. Some prior art failures are caused by plugging of the denitrification reactor, which causes backup into the nitrification field. If the latter occurs, the field becomes saturated (free draining aerobic conditions are required) and nitrification ceases.